Time-delay-and-integrate image sensors having variable intergration times

ABSTRACT

In various embodiments, a time-delay-and-integrate (TDI) image sensor includes (i) a plurality of integrating CCDs (ICCDs), arranged in parallel, that accumulate photocharge in response to exposure to light, (ii) electrically coupled to the plurality of ICCDs, a readout CCD (RCCD) for receiving photocharge from the plurality of ICCDs, and (iii) electrically coupled to the RCCD, readout circuitry for converting charge received from the RCCD into voltage.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/540,113, filed Sep. 28, 2011, U.S. ProvisionalPatent Application No. 61/540,117, filed Sep. 28, 2011, U.S. ProvisionalPatent Application No. 61/540,120, filed Sep. 28, 2011, and U.S.Provisional Patent Application No. 61/541,189, filed Sep. 30, 2011, theentire disclosure of each of which is hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates, in various embodiments, to theconstruction, fabrication, and use of time-delay-and-integrate (TDI)image sensors.

BACKGROUND

Electronic image-capture devices are typically divided broadly into twotypes: cameras and scanners. Cameras employ electronic image sensorsthat have a two-dimensional (i.e., areal) array of photosensitive areas(or “photosites”) that permit an image of a scene to be captured withoutrequiring relative motion between the scene to be captured, the imagesensor, and any optical elements used for forming an optical image ofthe scene on the image sensor. The photosites typically collectphoton-induced electrical charge (or “photocharge”) over some timeperiod, and the electrical charge is measured and transformed into thepicture elements (pixels) of the captured image. By way of example, theKODAK KAF-8300 is an areal-array image sensor for use in cameras. TheKAF-8300 includes a 3326×2504 two-dimensional array of photosites, eachof which separately collects photocharge, and which collectively provide8.3 million pixels in a captured image.

In contrast, scanners typically require relative motion between thescene to be captured and the image sensor, or movement of opticalelements used for forming an optical image on the sensor, to sweep theoptical image of the scene across the photosensitive areas of theelectronic image sensor. Scanners typically employ electronic imagesensors that have a one-dimensional (i.e., linear) array ofphotosensitive areas. Photocharge is allowed to accumulate over sometime period in the photosites, and the resulting accumulated charge ineach photosite is then measured. This accumulate-and-measure processoccurs repetitively during the scanning process, with each iterationforming a single line of pixels in the captured image. In this way, atwo-dimensional captured image is formed from successively capturedsingle lines of pixels. In one common scanner variant, multiple lineararrays, each array provided with a color-selective filter, are employedto capture color images. By way of example, the KODAK KLI-4104 is alinear-array image sensor for use in scanners. The KLI-4104 includesfour linear arrays: three separate arrays of 4080 10 μm photosites, witheach array filtered to capture red, green, or blue light, and a fourtharray of 8160 5 μm photosites that are unfiltered to permit capturelight over a broad color spectrum.

Scanners are used for capturing images of documents, for capturingimages of moving items in an manufacturing plant (for example, cannedbeverages), for robotic vision (typically employing a polygonal mirrorto sweep the scene image across the linear sensor), and for airplane- orsatellite-based imaging of the surface of the Earth. In some of theseapplications the image-capture device may be called a camera, but if theapplication requires relative motion between the scene and the imagesensor (or movement of optical elements to produce an effective motionof the scene across the image sensor) it employs a scanner as the termis used herein.

A time-delay-and-integrate (TDI) sensor is a particular type ofelectronic image sensor employed in scanners. In contrast to alinear-array sensor, a TDI sensor employs multiple photosites that areused collectively to form each pixel of the captured image. The multiplephotosites used for a given pixel are arranged in a column that isaligned with the direction of motion of the optical image across thesensor. In this way, each photosite in the column is presentedsequentially with a particular portion of the optical image. Thephotocharge accumulated in each successive photosite during the timethat the portion of the optical image moves over the photositecontributes to the respective pixel in the captured image. In thismanner, the TDI sensor increases the photocharge accumulation time foreach pixel of the captured image. A typical TDI sensor includes manycolumn-wise photosites arranged in parallel.

A TDI charge-coupled device (CCD) sensor 100 is shown in FIG. 1. The TDIsensor 100 includes multiple integrating CCDs (ICCDs) 102, a readout CCD(RCCD) 104, and a charge-measurement and amplifier circuit 106. (In somedescriptions, the ICCDs are called vertical CCDs and the RCCD is calleda horizontal CCD.) As an optical image sweeps vertically downward acrossthe ICCDs 102, the charge-shifting mechanism of the ICCDs is employed tomove packets of charge downward simultaneously with the movement of theoptical image. As a packet of charge moves from the top of an ICCD 102to the RCCD 104, it travels through multiple photosites and accumulatesadditional photocharge along the way. When a line of charge packetsreaches the RCCD 104, the charge packets are shifted laterally in theRCCD (in the illustrated embodiment) to be individually read out of thesensor 100 by the charge-measurement and amplifier circuit 106. The timerequired for a charge packet to travel from the top of an ICCD 102 untilit enters the RCCD 104 for readout is the exposure time, or integrationtime, for a given pixel of the captured image. Compared to aconventional linear-array image sensor, a TDI image sensor typicallyenables significantly increased integration time.

FIG. 2 further illustrates the operation of a TDI image sensor. Lightfrom scene element 202 is collected by optical system 204 in order toproduce an optical image 206 on the face of the TDI image sensor. Sceneelement 202 moves vertically upward with respect to optical system 204and the TDI image sensor. This causes corresponding optical image 206 tomove vertically downward across the surface of the TDI image sensor.Simultaneously with the downward motion of the optical image 206, theICCDs of the TDI image sensor are clocked downward toward the RCCD ofthe TDI image sensor. As each line of accumulated packets of photochargefrom the ICCDs is clocked vertically into the RCCD and then horizontallyout through the TDI image sensor's charge-measurement and amplifiercircuit, a line of pixels of the captured image is produced.

If a scene to be captured is sufficiently bright, allowing photochargeto accumulate over the length of the ICCD may cause the accumulatedphotocharge to exceed the charge capacity of the ICCD for the brightestareas of the scene. To avoid this, one of the horizontal clock lines forthe ICCDs may be held in a state to prevent charge packets from abovethe clock line from continuing below the clock line. For example,horizontal clock line 108 (see FIG. 1) may be used to block charge fromthe upper 15/16 of the ICCDs 102, effectively reducing the integrationtime to 1/16 of the potential full integration time. In similar fashion,horizontal clock line 110 reduces integration time to ⅛ of the fullintegration time, horizontal clock line 112 reduces integration time to¼ of the full integration time, and horizontal clock line 114 reducesintegration time to ½ of the full integration time.

By way of example of the type of image sensor shown in FIG. 1, a TDIimage sensor is described in “A High Speed, Dual Output Channel, StageSelectable, TDI CCD Image Sensor for High Resolution Applications”(Agwani, et al, Proc. SPIE, Vol. 2415, Page 124 (1995)). The devicedescribed has 2048 ICCDs, each ICCD consisting of 96 CCD integratingstages, and with the number of integrating stages selectable among 96,48, 24, 12, and 6 stages. The RCCD is split into two CCDs, with one CCDfor even-numbered ICCDs and the other for odd-numbered ICCDs, and withseparate charge measurement and amplifier circuits associated with eachof the two readout CCDs. The sensor provides captured image pixel linesat up to 14,000 lines per second, and provides a dynamic range of6000:1.

Although TDI CCD image sensors generally have very high sensitivity dueto the long integration times provided by the ICCDs and also haveflexibility in integration time by selecting the number of stages ofintegration employed, there remains a need for greater dynamic range.For example, when capturing images of the Earth's surface, a naturalbody of water or water standing on the roof of a building may reflectsunlight, while nearby scene elements may be dark or in shadow. In sucha situation, the range of light level between the reflected sunlight andthe dark areas of the scene may far exceed the 6000:1 dynamic range of atypical CCD TDI image sensor such as the one described above.

One proposed technique for increasing dynamic range in a TDI CCD isdescribed in “An Adaptive Sensitivity™ TDI CCD Sensor” (Chen andGinosar, Proc. SPIE, Vol. 2950, 45 (1996)). In this sensor, each ICCD iscomposed of 13 TDI stages, a conditional reset circuit, 4 more TDIstages, another conditional reset circuit, and a final TDI stage beforereaching the RCCD. The conditional reset circuits include acharge-measurement amplifier that controls a discharge gate: as eachcharge packet is clocked through the CCD stage associated with theconditional reset circuit, the amount of charge is measured. If themeasured charge exceeds a threshold, the discharge gate is operated toremove the charge from the CCD. In this fashion, the dynamic range ofthe image sensor is increased: dark areas of the scene do not causeeither of the conditional reset circuits in an ICCD to trigger, therebygetting the benefit of the full 13+4+1=18 TDI stages; middle brightnessareas of the scene cause the first conditional reset circuit to trigger,but not the second, allowing the use of 4+1=5 TDI stages; and thebrightest areas of the scene cause both conditional reset circuits totrigger, thereby using only a single TDI stage to capture those areas ofthe scene. Effectively this increases the dynamic range of the sensor bya factor of 18, i.e., the difference between 18 TDI stages used for darkareas of the scene and 1 TDI stage used for bright areas of the scene.

However, there are several shortcomings with this approach. First, theconditional reset circuit consumes a significant amount of area, as itincludes multiple transistors. Second, a contact must be placed in theCCD stage associated with the conditional reset circuit to permit themeasurement of charge, and the contact has the potential for producingdark current or otherwise affecting the charge packet as it passesthrough the affected CCD stage. Additionally, there is no mechanism fordetermining from the output whether a particular pixel integrated overthe full 18 stages, was reset once and integrated over only 4 stages, orwas reset twice and integrated over only a single stage. Therefore,there remains a need to increase the dynamic range of a TDI CCD imagesensor while addressing these shortcomings.

SUMMARY

Embodiments of the present invention increase the dynamic range of TDICCD image sensors by selectively controlling the projection of lightonto the image sensor and/or selectively resetting one or more stages ofone of more of the ICCDs of the image sensor while and/or prior to ascene being imaged by the TDI image sensor. In preferred embodiments thestages are substantially identical to each other and are allindependently resettable, in contrast to more complex conventionaldesigns utilizing only a few specialized resettable stages. Thedynamic-range control of the image sensor may be based on previouslycaptured images, which may be captured either by the image sensor itselfor another image sensor (termed a “leading sensor”) that is configuredto capture light from a scene immediately prior to the scene beingimaged by the TDI image sensor. (This prior-captured light may be termeda “leading image,” which as used herein refers to at least a portion ofa scene to be imaged by the TDI image sensor and which may be at adifferent resolution than the scene as later captured by the TDI imagesensor.) For example, bright areas may be identified prior to imaging,and stages of the image sensor corresponding to such areas may beselectively reset to prevent “charge blooming” into neighboring ICCDsduring image capture. Alternatively or in combination, the amount oflight from the scene reaching stages of the image sensor correspondingto the bright areas may be decreased via use of, e.g., an optical mask,thereby substantially preventing the generation of excessive photochargein those stages. Each of the ICCDs may incorporate a sense node formeasurement of charge within the ICCD channel and/or selective reset ofstages of the ICCDs via direction of photocharge into the sense node.Alternatively or in combination, all of the ICCD stages may beconfigured for individual reset via application of a bias to the stagecontrol line (each of which preferably controls a particular stagecommon across all of the ICCDs) and to a gate associated with each ICCD.As used herein, a “scene” does not connote any particular content, andmay be, e.g., a pictorial scene, a graphical scene (e.g., a document orother text), a medical image, etc.

In an aspect, embodiments of the invention feature an imaging systemthat includes a time-delay-and-integrate (TDI) image sensor including orconsisting essentially of (i) a plurality of integrating CCDs (ICCDs),arranged in parallel, that accumulate photocharge in response toexposure to light, (ii) electrically coupled to the plurality of ICCDs,a readout CCD (RCCD) for receiving photocharge from the plurality ofICCDs, and (iii) electrically coupled to the RCCD, readout circuitry forconverting charge received from the RCCD into voltage. An optical systemreceives light from a scene to be imaged and projects it on theplurality of ICCDs. A leading sensor receives the light projected fromthe optical system prior to projection thereof on the plurality ofICCDs, thereby capturing a leading image of the scene. A control systemcontrols operation of the TDI image sensor based at least in part on atleast a portion of the leading image.

Embodiments of the invention may feature one or more of the following inany of a variety of combinations. The optical system may be configuredto sweep the light from the scene across the leading sensor and the TDIimage sensor continuously and sequentially. The leading sensor may beconfigured to collect color information related to the scene. The TDIimage sensor may not be configured to collect color information relatedto the scene or may be configured to collect monochrome intensity levels(e.g., only monochrome intensity levels) related to the scene. Theleading sensor and the TDI image sensor may be disposed on a substrate.The dynamic range of the leading sensor may be less than the dynamicrange of the TDI image sensor. The control system may alter theintegration time of at least one of the ICCDs based on the brightnesslevel of at least a portion of the leading image. The TDI image sensormay include an optical mask disposed between the optical system and theplurality of ICCDs. The optical mask may include or consist essentiallyof an array of masking elements, and the control system may control themasking elements to mask portions of the ICCDs whereby light collectionat such portions is attenuated. The optical mask may include or consistessentially of an array of reflective elements, and the control systemmay control the reflective elements to selectively reflect portions ofthe light from the optical system onto the plurality of ICCDs.

Each ICCD may include or consist essentially of (i) a plurality ofindependently controllable stages, (ii) a photosensitive channel forcontaining photocharge, (iii) a drain for removing photocharge from thechannel, and (iv) a gate for controlling flow of photocharge from thechannel to the drain. A plurality of clock lines may be disposedsubstantially perpendicular to the ICCDs. Each clock line may control aparticular stage common to all of the ICCDs. The control system may beconfigured to reset a selected stage of a selected ICCD by (i) applyinga bias to the clock line corresponding to the selected stage and (ii)applying a bias to the gate corresponding to the selected ICCD.

Each ICCD may include or consist essentially of (i) a plurality ofindependently controllable stages, (ii) a photosensitive channel forcontaining photocharge, (iii) a sense node for measuring photochargereceived thereby from the channel, and (iv) a gate for controlling flowof photocharge from the channel to the sense node. The control systemmay be configured to (i) measure photocharge received by the sense nodefrom the channel and (ii) reset the sense node by applying a biasthereto to remove photocharge therefrom. The control system may beconfigured to reset a selected stage of the ICCD by applying a bias tothe gate to thereby allow photocharge to flow from the channel into thesense node.

In another aspect, embodiments of the invention feature a method ofimage capture utilizing a time-delay-and-integrate (TDI) image sensorthat includes or consists essentially of (i) a plurality of integratingCCDs (ICCDs), arranged in parallel, that accumulate photocharge inresponse to exposure to light, (ii) electrically coupled to theplurality of ICCDs, a readout CCD (RCCD) for receiving photocharge fromthe plurality of ICCDs, and (iii) electrically coupled to the RCCD,readout circuitry for converting charge received from the RCCD intovoltage. Light received from a scene to be imaged is projected onto aleading sensor, the leading sensor capturing a leading image of thescene. Thereafter, light from the scene is projected onto the pluralityof ICCDs to capture a scene image. During capture of the scene image,operation of the TDI sensor is controlled based at least in part on atleast a portion of the leading image.

Embodiments of the invention may feature one or more of the following inany of a variety of combinations. The projection of light onto theleading sensor and onto the plurality of ICCDs may be performedcontinuously and sequentially. The leading image may include colorinformation. The scene image may be substantially free of colorinformation. Controlling operation of the TDI sensor may include orconsist essentially of altering an integration time of at least one ofthe ICCDs based on the brightness level of at least a portion of theleading image. Controlling operation of the TDI sensor may include orconsist essentially of masking portions of the ICCDs whereby lightcollection at such portions is attenuated. Controlling operation of theTDI sensor may include or consist essentially of resetting a selectedstage of a selected ICCD. Resetting the selected stage may include orconsist essentially of transferring photocharge therewithin into a drainassociated with the selected ICCD. Resetting the selected stage mayinclude or consist essentially of transferring photocharge therewithininto a sense node associated with the selected ICCD. The leading imagemay be compared with the scene image to detect motion in the scene.

In yet another aspect, embodiments of the invention feature an imagingsystem including or consisting essentially of a time-delay-and-integrate(TDI) image sensor, an optical system for receiving light from a sceneto be imaged, and an optical mask. The TDI image sensor includes orconsists essentially of (i) a plurality of integrating CCDs (ICCDs),arranged in parallel, that accumulate photocharge in response toexposure to light, (ii) electrically coupled to the plurality of ICCDs,a readout CCD (RCCD) for receiving photocharge from the plurality ofICCDs, and (iii) electrically coupled to the RCCD, readout circuitry forconverting charge received from the RCCD into voltage. The opticalsystem projects light from the scene to be imaged on the plurality ofICCDs. The optical mask is disposed between the optical system and theplurality of ICCDs and selectively alters the intensity of lightprojected to at least portions of the ICCDs.

Embodiments of the invention may feature one or more of the following inany of a variety of combinations. The optical mask may include orconsist essentially of an array of independently controllable maskingelements each for attenuating light collection by a different portion ofthe ICCDs. The optical mask may include or consist essentially of anarray of independently controllable reflective elements each forselectively reflecting a portion of the light from the optical systemonto the ICCDs. A control system may control the optical mask based atleast in part on light from the scene to be imaged before such light isprojected by the optical system.

In a further aspect, embodiments of the invention feature a method ofimage capture utilizing a time-delay-and-integrate (TDI) image sensorcomprising (i) a plurality of integrating CCDs (ICCDs), arranged inparallel, that accumulate photocharge in response to exposure to light,(ii) electrically coupled to the plurality of ICCDs, a readout CCD(RCCD) for receiving photocharge from the plurality of ICCDs, and (iii)electrically coupled to the RCCD, readout circuitry for convertingcharge received from the RCCD into voltage. Light received from a sceneto be imaged is projected onto the plurality of ICCDs to capture animage of the scene. During capture of the image, the intensity of lightprojected to at least portions of the ICCDs is selectively altered.

Embodiments of the invention may feature one or more of the following inany of a variety of combinations. The intensity of light projected to atleast portions of the ICCDs may be altered with an optical mask disposedbetween the scene and the ICCDs. The optical mask may include or consistessentially of an array of masking elements each independentlycontrollable to mask a portion of the ICCDs whereby light collection inthe masked portion is attenuated. The optical mask may include orconsist essentially of an array of reflective elements eachindependently controllable to reflect a portion of the projected lightonto the ICCDs. The selective alteration of the intensity of lightprojected to at least portions of the ICCDs during capture of the imagemay be based at least in part on a previously captured image.

In another aspect, embodiments of the invention feature an imagingsystem including or consisting essentially of a time-delay-and-integrate(TDI) image sensor and an optical system for receiving light from ascene to be imaged. The TDI image sensor includes or consistsessentially of (i) a plurality of integrating CCDs (ICCDs), arranged inparallel, that accumulate photocharge in response to exposure to light,(ii) electrically coupled to the plurality of ICCDs, a readout CCD(RCCD) for receiving photocharge from the plurality of ICCDs, and (iii)electrically coupled to the RCCD, readout circuitry for convertingcharge received from the RCCD into voltage. The optical system projectslight from the scene to be imaged on the plurality of ICCDs. Each ICCDincludes or consists essentially of (i) a plurality of independentlycontrollable stages, (ii) a photosensitive channel for containingphotocharge, (iii) a drain for removing photocharge from the channel,and (iv) a gate for controlling flow of photocharge from the channel tothe drain.

Embodiments of the invention may feature one or more of the following inany of a variety of combinations. A plurality of clock lines may bedisposed substantially perpendicular to the ICCDs. Each clock line maycontrol a particular stage common to all of the ICCDs. A control systemmay be configured to reset a selected stage of a selected ICCD by (i)applying a bias to the clock line corresponding to the selected stageand (ii) applying a bias to the gate corresponding to the selected ICCD.The control system may be configured to reset the selected stage basedat least in part on a previously captured image (i.e., an image capturedby the imaging system prior to the light from the scene to be imagedbeing projected by the optical system).

In yet another aspect, embodiments of the invention feature a method ofimage capture utilizing a time-delay-and-integrate (TDI) image sensorcomprising (i) a plurality of integrating CCDs (ICCDs), arranged inparallel and each comprising a plurality of stages, that accumulatephotocharge in response to exposure to light, (ii) electrically coupledto the plurality of ICCDs, a readout CCD (RCCD) for receivingphotocharge from the plurality of ICCDs, and (iii) electrically coupledto the RCCD, readout circuitry for converting charge received from theRCCD into voltage. Each stage of each ICCD is independently resettable.Light received from a scene to be imaged is projected onto the pluralityof ICCDs to capture an image of the scene. During capture of the image,a selected stage of a selected ICCD is reset to remove photocharge fromthe selected stage. Resetting the selected stage may include or consistessentially of transferring photocharge therewithin into a drainassociated with the selected ICCD.

In an additional aspect, embodiments of the invention feature an imagingsystem including or consisting essentially of a time-delay-and-integrate(TDI) image sensor and an optical system for receiving light from ascene to be imaged. The TDI image sensor includes or consistsessentially of (i) a plurality of integrating CCDs (ICCDs), arranged inparallel, that accumulate photocharge in response to exposure to light,(ii) electrically coupled to the plurality of ICCDs, a readout CCD(RCCD) for receiving photocharge from the plurality of ICCDs, and (iii)electrically coupled to the RCCD, readout circuitry for convertingcharge received from the RCCD into voltage. The optical system projectslight from the scene to be imaged on the plurality of ICCDs. Each ICCDincludes or consists essentially of (i) a plurality of independentlycontrollable stages, (ii) a photosensitive channel for containingphotocharge, (iii) a sense node for measuring photocharge receivedthereby from the channel, and (iv) a gate for controlling flow ofphotocharge from the channel to the sense node.

Embodiments of the invention may feature one or more of the following inany of a variety of combinations. A plurality of clock lines may bedisposed substantially perpendicular to the ICCDs. Each clock line maycontrol a particular stage common to all of the ICCDs. A control systemmay be configured to (i) measure photocharge received by the sense nodefrom the channel and (ii) reset the sense node by applying a biasthereto to remove photocharge therefrom. A control system may beconfigured to reset a selected stage of the ICCD by applying a bias tothe gate to thereby allow photocharge to flow from the channel into thesense node. The control system may be configured to reset the selectedstage based at least in part on a previously captured image (i.e., animage captured by the imaging system prior to the light from the sceneto be imaged being projected by the optical system).

In yet an additional aspect, embodiments of the invention feature amethod of image capture utilizing a time-delay-and-integrate (TDI) imagesensor comprising (i) a plurality of integrating CCDs (ICCDs), arrangedin parallel, that accumulate photocharge in response to exposure tolight, (ii) electrically coupled to the plurality of ICCDs, a readoutCCD (RCCD) for receiving photocharge from the plurality of ICCDs, and(iii) electrically coupled to the RCCD, readout circuitry for convertingcharge received from the RCCD into voltage. Each ICCD includes orconsists essentially of (i) a plurality of independently controllablestages, (ii) a photosensitive channel for containing photocharge, (iii)a sense node for measuring photocharge received thereby from thechannel, and (iv) a gate for controlling flow of photocharge from thechannel to the sense node. Light received from a scene to be imaged isprojected onto the plurality of ICCDs. During light projection, (i)photocharge received by the sense node from the channel is measured and(ii) the sense node is reset by applying a bias thereto to removephotocharge therefrom, and/or during light projection, a selected stageof the ICCD is reset by applying a bias to the gate to thereby allowphotocharge to flow from the channel into the sense node.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 illustrates a conventional TDI CCD image sensor;

FIG. 2 illustrates an exemplary operation mode of a TDI CCD imagesensor;

FIG. 3A is a block diagram of an image-capture device utilizing an imagesensor in accordance with various embodiments of the invention;

FIG. 3B is a block diagram of portions of an image sensor in accordancewith various embodiments of the invention;

FIG. 4 is a schematic plan view of portions of two ICCDs of a TDI CCD inaccordance with various embodiments of the invention;

FIG. 5 is a schematic cross-section of a vertical portion of the ICCDsof FIG. 4 showing ICCD clock lines and associated potential diagrams inaccordance with various embodiments of the invention;

FIG. 6 is a schematic cross-section of a horizontal portion of the ICCDsof FIG. 4 showing two ICCD integrating channels with associatedstructures in accordance with various embodiments of the invention;

FIG. 7 is a potential diagram for an unselected row of the ICCD stagesof FIG. 6 in accordance with various embodiments of the invention;

FIG. 8 is a potential diagram for a selected row of the ICCD stages ofFIG. 6 in accordance with various embodiments of the invention;

FIG. 9 expands the potential diagrams of FIG. 5 to depict four-phaseclocking for multiple ICCD stages in accordance with various embodimentsof the invention;

FIG. 10 expands the potential diagrams of FIG. 5 to depict four-phaseclocking for multiple ICCD stages including two different rows selectedfor reset in accordance with various embodiments of the invention;

FIG. 11 expands the potential diagrams of FIG. 10 to depict four-phaseclocking for multiple ICCD stages including four different rows selectedfor reset in accordance with various embodiments of the invention;

FIG. 12 is a potential diagram for an unselected row of the ICCD stagesof FIG. 6 in accordance with various embodiments of the invention;

FIG. 13 is a potential diagram for an unselected row of the ICCD stagesof FIG. 6 in accordance with various embodiments of the invention;

FIG. 14 is a modification of the potential diagrams of FIG. 11 depictingfour-phase clocking for multiple ICCD stages including two differentrows selected for reset in accordance with various embodiments of theinvention;

FIG. 15 is a potential diagram for individual gates of ICCD stages inaccordance with various embodiments of the invention;

FIG. 16 is a schematic plan view of a portion of an ICCD of a TDI CCD inaccordance with various embodiments of the invention;

FIG. 17 is a schematic cross section of a horizontal portion of the ICCDof FIG. 16 depicting an ICCD integrating channel with associatedstructures in accordance with various embodiments of the invention;

FIG. 18 is a potential diagram for an unselected gate of the ICCD ofFIG. 17 in accordance with various embodiments of the invention;

FIG. 19 is a potential diagram depicting charge spillover for anunselected gate of the ICCD of FIG. 17 in accordance with variousembodiments of the invention;

FIG. 20 is a potential diagram depicting charge discharge for a selectedgate of the ICCD of FIG. 17 in accordance with various embodiments ofthe invention;

FIG. 21 is a potential diagram depicting an unselected gate and nocharge in an ICCD integrating channel of the ICCD of FIG. 17 inaccordance with various embodiments of the invention;

FIG. 22 is a circuit diagram of a spillover charge-measurement circuitin accordance with various embodiments of the invention;

FIG. 23 is a schematic cross-section of a system in which a travellingoptical mask is in virtual contact with a sensor in accordance withvarious embodiments of the invention;

FIG. 24 is a schematic cross-section of a system in which an imagepassing through a travelling optical mask is relayed to a sensor inaccordance with various embodiments of the invention;

FIG. 25 is a schematic cross-section of a system in which an imagereflected from a travelling optical mask is relayed to a sensor inaccordance with various embodiments of the invention; and

FIG. 26 illustrates an exemplary operation mode of a TDI sensor with aleading sensor in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

FIG. 3A is a block diagram of an exemplary image-capture device 300 thatincludes an electronic image sensor 304 in accordance with variousembodiments of the invention. In FIG. 3A, incoming light 302 from ascene to be captured is focused on image sensor 304 by optical system204. Image sensor 304 provides an analog electronic signal that isrepresentative of the optical image focused on the image sensor. Thisanalog signal is processed by an analog signal processor 306. The analogsignal processor 306 typically performs one or more of the functions ofsignal sampling, reference/signal correlation, gain, black-leveladjustment, and other analog processing. The output of the analog signalprocessor 306 is converted to digital output 310 by analog-to-digital(A/D) converter 308. A timing generator synchronizes the operation ofthe image sensor 304, the analog signal processor 306, and the A/Dconverter 308. Two or more elements of image-capture device 300 may becombined into a single device: for example, the analog signal processor304, the A/D converter 308, and the timing generator 312 may be combinedinto a single integrated circuit. Conversely, an element ofimage-capture device 300 may be composed of multiple devices: forexample, the timing generator 312 may be composed of an integratedcircuit that provides logic-level timing signals and other integratedcircuits and/or support circuitry that convert some of the logic-leveltiming signals into signals appropriate for driving the image sensor.The image sensor 304 may be an areal-array sensor, a linear-arraysensor, a TDI sensor, or other electronic image sensor. The image sensor304 typically employs CCD technology, active pixel sensor (i.e., CMOSimage sensor) technology, or other image-sensor technology.

Referring back to FIG. 1, the TDI CCD image sensor 100 includes multipleICCDs 102, RCCD 104, and charge-measurement and amplifier circuit 106.In general, embodiments of the present invention employ similarstructures. However, various embodiments of the present inventionprovide improvements to the structure and operation of the ICCDs 102.

The ICCD 102 is considered to be a charge-coupled device because itenables packets of charge to be maintained separately from each otherand also enables the packets of charge to be shifted along the length ofthe ICCD. The ICCD 102 is also considered to be an integrating devicebecause photon-induced charge (photocharge) increases the amount ofcharge in a charge packet during the period of time the charge packetremains in the ICCD. By collecting photocharge into packets of chargeduring the time the packets are shifted along its length, the ICCD 102permits significantly increased integration time and consequentlysignificantly increased sensitivity.

FIG. 3B schematically depicts image sensor 304 linked to a controller(or “control system”) 314 that controls various operations of imagesensor 304, including image capture and read out (and otherfunctionality described below), as well as various operations of othercomponents of image-capture systems described herein. The controller 314(which in various embodiments of the invention includes or performs thefunctionalities of the analog signal processor 306, the A/D converter308, and the timing generator 312) may be a general-purposemicroprocessor, but depending on implementation may alternatively be amicrocontroller, peripheral integrated circuit element, acustomer-specific integrated circuit (CSIC), an application-specificintegrated circuit (ASIC), a logic circuit, a digital signal processor,a programmable logic device such as a field-programmable gate array(FPGA), a programmable logic device (PLD), a programmable logic array(PLA), an RFID processor, smart chip, or any other device or arrangementof devices that is capable of implementing the steps of the processes ofthe invention (such as those described in detail below). The controller314 may be monolithically integrated with, and thus a portion of thesame integrated-circuit chip as, image sensor 304, or controller 314 maybe disposed on a chip separate and discrete from the chip containingimage sensor 304 (and interconnected thereto by wired or wirelessmeans). Moreover, at least some of the functions of controller 314 maybe implemented in software and/or as mixed hardware-software modules.Software programs implementing the functionality herein described may bewritten in any of a number of high level languages such as FORTRAN,PASCAL, JAVA, C, C++, C#, BASIC, various scripting languages, and/orHTML. Additionally, the software may be implemented in an assemblylanguage directed to a microprocessor resident in controller 314. Thesoftware may be embodied on an article of manufacture including, but notlimited to, a floppy disk, a jump drive, a hard disk, an optical disk, amagnetic tape, a PROM, an EPROM, EEPROM, field-programmable gate array,CDROM, or DVDROM. Embodiments using hardware-software modules may beimplemented using, for example, one or more FPGA, CPLD, or ASICprocessors.

A plan view of portions of two adjacent ICCDs 400 of a TDI CCD in anembodiment of the present invention is shown in FIG. 4. FIG. 5 includesa cross-section of a vertical portion of one of the ICCDs 400, as shownby the cut line 5-5 in FIG. 4. FIG. 6 is a cross-section that cutshorizontally through both of the ICCDs 400, as shown by cut line 6-6 inFIG. 4.

The schematic cross-section at the top of FIG. 5 depicts one stage of anICCD 400 in accordance with an embodiment of the present invention. FourICCD clock lines are shown: ICCD clock line 1 (502), ICCD clock line 2(504), ICCD clock line 3 (506), and ICCD clock line 4 (508). Voltagesare applied sequentially to these ICCD clock lines in order to producepotential gradients in the underlying semiconductor, during whichpackets of charge are kept separate and are moved along the length ofthe ICCD 400. The four clock lines 502, 504, 506, 508 are operated infour phases to hold and move charge in a manner well understood by thoseskilled in the art of CCD design and operation. In FIG. 5 the potentialdiagrams Φ1, Φ2, Φ3, and Φ4 illustrate the potentials developed undereach of the four ICCD clock lines for each of the four phases used innormal operation of ICCD 400. In potential diagram Φ1, a charge packet510 is held under ICCD clock line 1 (502) and ICCD clock line 2 (504),and the charge packet is separated from other charge packets by ICCDclock line 3 (506) and ICCD clock line 4 (508). This arrangement of ICCDclock lines and potentials is repeated over the length of the ICCD 400so that, for example, the ICCD clock line immediately to the left ofICCD clock line 1 is electrically connected to and driven to the samevoltage as ICCD clock line 4, thereby acting to contain the chargepacket 510 on its left side. (Note that the charge packet 510 in atypical CCD is composed of electrons, so a more positive voltagepotential produces an area for electrons to collect. The potentialdiagrams of FIG. 5 and other figures are drawn to show electron chargebeing collected in low areas in order to facilitate an understanding ofthe CCD operation by analogy to fluids.) Thus, in potential diagram Φ1,ICCD clock lines 1 (502) and 2 (504) produce a storage region forholding a charge packet, and ICCD clock lines 3 (506) and 4 (508)produce a barrier region for separating charge packets. In potentialdiagram Φ2, the charge packet is moved to the right (or verticallydownward with respect to FIG. 4) by changing the voltages on ICCD clocklines 1 (502) and 3 (506). Similarly, potential diagrams Φ3 and Φ4 movethe charge packet two more clock lines to the right. By then returningto the potential diagram Φ1, a new charge packet is held in the storageregion produced by ICCD clock lines 1 (502) and 2 (504). Herein,potential diagrams Φ1 through Φ4 are referred to alternatively as phaseΦ1 through Φ4.

The four ICCD clock lines 502, 504, 506, 508 together constitute onestage of an ICCD 400. The arrangement in FIG. 5 is repeated to produce acomplete ICCD 400 that has, for example, 32, 48, 64, or 96 stages. As acharge packet is clocked along the ICCD 400 in the manner describedabove, photocharge is collected in the storage regions at a rate thatdepends on the amount of light incident on the sensor due to the movingoptical image formed on the sensor. A charge packet moving along theICCD 400 and accumulating photocharge along the way is an “integratingpixel;” when an integrating pixel is read out from the ICCD 400 itbecomes a pixel of the capture image.

FIG. 5 includes a potential diagram Φ2′ that is an alternative topotential diagram Φ2. In potential diagram Φ2′, the voltages applied tothe ICCD clock lines 502, 504, 506, 508 are adjusted to shift thebarrier and storage regions of the corresponding ICCD stage with respectto adjacent stages. This shifting is shown by the arrows in potentialdiagram Φ2′. In an embodiment of the invention, the shifting occurs incommon for the corresponding stage in all the ICCDs 400 of the TDI imagesensor: in reference to FIG. 1, this defines a horizontal row of ICCDstages, one stage from each ICCD 102 in FIG. 1. By shifting thepotential for a row of ICCD stages in this way, the corresponding row ofintegrating pixels is selected for possible reset as described below.This shifting of the potential of a row of ICCD stages is referred toherein as “selecting” a row of ICCD stages or defining a “selected row.”

The cross-section in FIG. 6 cuts along one ICCD clock line 602 andacross two ICCDs 400 constructed in accordance with an embodiment of theinvention. Specifically, the cross-section in FIG. 6 cuts along ICCDclock line 2 (504 in FIG. 5). Channel stop implants 604 in FIG. 6produce potential barriers to separate each ICCD 400 from adjacent ICCDs400. Drain implants 606 produce potential regions that permit removal ofcharge from the adjacent ICCD 400. Gates 608 produce potential regionsthat provide a barrier between an ICCD 400 and its corresponding drainimplant 606; the gates 608 are individually externally controllable toeliminate the barrier between an ICCD 400 and its corresponding drainimplant 606, thereby allowing charge to flow from the ICCD 400 into thedrain implant 606 under certain conditions.

FIG. 7 is a potential diagram associated with FIG. 6 for an unselectedrow of an ICCD in accordance with an embodiment of the invention. InFIG. 7, the ICCD clock lines are driven to produce the potentialsassociated with potential diagram Φ2 in FIG. 5. As shown, the gate 608associated with the left ICCD (left 608 in FIG. 6) remains in a statethat produces a barrier between the left ICCD and its correspondingdrain 606 (left 606 in FIG. 6), thereby preventing the charge packet 702within the integrating pixel thus defined from flowing into the drain606. The gate 608 associated with the right ICCD (right 608 in FIG. 6)is driven so that the barrier between the right ICCD and itscorresponding drain 606 (right 606 in FIG. 6) is reduced; however, thebarrier is still sufficient to prevent the charge packet 704 within thatintegrating pixel from flowing into the drain 606. Thus, the potentialdiagram Φ2 in FIG. 5 prevents an integrating pixel from being reset (thecharge packet being drained away) regardless of the states of the gatesassociated with the ICCDs. Hence, potential diagram Φ2 in FIG. 5represents an unselected row of ICCD stages that may prevent integratingpixels from being reset regardless of whether an ICCD is selected (e.g.,the right ICCD in FIG. 7) or unselected (e.g., the left ICCD in FIG. 7).

FIG. 8 is a potential diagram associated with FIG. 6 for a selected ICCDrow in accordance with an embodiment of the invention. In FIG. 8, theICCD clock lines are thus driven to produce the potentials associatedwith potential diagram Φ2′ in FIG. 5. In FIG. 8, even though thepotential of the storage region has been shifted, the barrier producedby the gate 608 associated with the left ICCD (left 608 in FIG. 6) issufficient to prevent the charge packet 802 from flowing into the drain606. The charge packet 802 is effectively held by a storage regiondefined by the potentials associated with ICCD clock lines 1 and 4 (502and 508 in FIG. 5), the gate 608 associated with the left ICCD (left 608in FIG. 6), and the channel stop implant 604 associated with the leftICCD (leftmost 604 in FIG. 6). However, since the potential of thestorage region has been shifted, the reduced barrier produced by thegate 608 associated with the right ICCD (right 608 in FIG. 6) is nowlower than the floor of the storage region in the right ICCD, therebyallowing the charge packet 804 to flow into the corresponding drain 606.Thus, as shown, the concurrent selection of a row of ICCD stages alongwith a selected ICCD will cause the integrating pixel from thecorresponding storage region to be reset (i.e., the charge packetconfined therein to flow to the drain). This effectively reduces theavailable integration time for the integrating pixel, after being reset,to be the time spent in the remaining ICCD stages between where theintegrating pixel reset occurred and the end of the ICCD.

In the embodiment described above, any integrating pixel may be reset atany point as it is clocked along its corresponding ICCD by selection ofthe appropriate ICCD clock lines (thereby selecting a row of ICCDstages) and ICCD gate (thereby selecting a particular ICCD). Theselective reset of integrating pixels provided by this arrangementenables integrating pixels associated with bright regions of the sceneto be reset at a point close to the end of the ICCD, thereby permittingthem to have a short effective integration time. In a similar fashion,integrating pixels associated with middle ranges of scene brightness areenabled to be reset in the middle of the ICCD, permitting them to have amedium length effective integration time. Integrating pixels associatedwith dark areas of the scene may not be reset at all, permitting them tohave the maximum integration time enabled by all stages of the ICCD.This has the effect of significantly increasing the dynamic range of theTDI image sensor in accordance with embodiments of the presentinvention.

The embodiments of the invention described above provide the advantageof increased dynamic range without the shortcomings of earlierapproaches. For example, compared to conventional charge-measurementcircuitry, the drain and gate structures associated with each ICCD arecompact structures. All of the ICCD stages are typically identical inthe disclosed embodiment, and thus no specialized stages incorporatingcharge-measurement and discharge circuitry are required. Embodiments ofthe present invention also do not require a contact to be placed in theICCD for charge measurement, thereby eliminating a potential source ofdark current or other undesirable effects. Finally, since the ICCD clocklines and, in particular, the ICCD gates are externally controllable,the controller that controls these elements and thereby causes the resetof integrating pixels has knowledge of which integrating pixels werereset and at which ICCD stage they were reset. Consequently, in contrastto the output pixels produced by conventional sensors, there is noambiguity regarding the output pixels: since the reset conditions for aparticular integrating pixel are known, the integration time for thecorresponding output pixel is also known.

“Externally controllable,” as used in the foregoing discussion, meansthat the determination regarding which integrating pixels to reset andat which ICCD stage to reset them occurs separately from the ICCDs andin such a way that the knowledge of which integrating pixels were resetand at which stage they were reset is available for subsequentimage-processing purposes. Externally controllable is not necessarilyequivalent to “directly controlled,” as there may be interveningcircuitry such as decoders, control registers, shift registers, or thelike between the controller that resets the integrating pixel and theICCD clock lines and gates. Externally controllable means external tothe array of ICCDs and does not imply that control elements must be on aseparate integrated circuit substrate; all or a portion of the elementsof the external control may be integrated into the same integratedcircuit substrate as the ICCDs, they may be incorporated into anintegrated circuit substrate that has been bonded to the TDI sensor, orthey may be separate.

Although the embodiment described above enables any integrating pixel tobe reset at any stage in the ICCD, such flexibility is not necessarilyrequired. For example, for an ICCD having n stages, providing selectivereset of integrating pixels at the (n−1) stage, the (n−2) stage, the(n−4) stage, and the (n−8) stage in an ICCD wherein n=16 permitsintegrating pixels reset at those stages to have, respectively, onestage of integration, two stages of integration, four stages ofintegration, or eight stages of integration. Integrating pixels that arenot reset at any of the sixteen stages have sixteen stages ofintegration. In this example, the selective reset of integrating pixelsincreases the dynamic range of the image sensor by a factor of 16. Thisarrangement has the additional benefit of restricting to four stages thedifferent clocking utilized to select a row of ICCD stages for possibleintegrating-pixel reset. This is discussed in more detail below.

FIG. 9 expands the potential diagrams from FIG. 5 to depict four-phaseclocking for five ICCD stages. As shown in FIG. 9, operating the ICCDclock lines to produce the potentials shown in potential diagrams Φ1through Φ4 (and back again to potential diagram Φ1) moves a chargepacket exactly one stage to the right. Each cycle through potentialdiagrams Φ1 through Φ4 in FIG. 9 moves charge packets one stage to theright. For convenience, the ICCD potential profiles related to thepotential diagrams Φ1 through Φ4 in FIG. 9 and subsequent figures arealso referred to as phases Φ1 through Φ4.

In FIG. 9, no rows of ICCD stages are selected for possible integratingpixel reset in any of the phases Φ1 through Φ4. By contrast, FIG. 10shows the same five ICCD stages and similar phase clocking as in FIG. 9,but phase Φ2 is modified in the same manner as in FIG. 5 in order toselect integrating pixels one stage from the right for possible resetand phase Φ1 is modified in order to select integrating pixelsapproximately two stages from the right for possible reset. For thefollowing discussion, the rightmost stage of FIGS. 9, 10, and 11 isassumed to be in communication with the RCCD and that the integratingpixel is read out of the rightmost stage into the RCCD during phase Φ3.By selectively operating ICCD gates coincident with phase Φ2, selectedintegrating pixels are reset one stage from the right and aresubsequently permitted to integrate only during the subsequent sequenceof four phases Φ3, Φ4, Φ1, and Φ2 required to transport the integratingpixels through the rightmost ICCD stage. Similarly, by selectivelyoperating the ICCD gates coincident with phase Φ1, selected integratingpixels are reset approximately two stages from the right and aresubsequently permitted to integrate only during the subsequent ninephases required to transport the integrating pixels through therightmost two ICCD stages.

Note that in order to permit integrating pixels two stages from theright to be reset independently of integrating pixels one stage from theright, the corresponding row of ICCD stages are selected duringdifferent stages. Consequently, as is seen in phase Φ1 of FIG. 10, theaffected ICCD clock lines are not precisely associated with the thirdstage from the right but are shifted leftward to include one ICCD clockline from the fourth stage.

The ICCD clock lines associated with the selected stage are controlledindependently from the other clock lines to permit different voltagelevels to be applied to them. In FIG. 10, ICCD clock lines 1A, 2A, 3A,and 4A for the second stage from the right are independent (i.e.,independently controllable) from the other ICCD clock lines to permitthe second stage from the right to be selected. Similarly, ICCD clockline 4B for the fourth stage from the right and clock lines 1B, 2B, and3B for the third stage from the right are independent from the otherICCD clock lines. ICCD clock lines 1, 2, 3, and 4 are typicallyconnected together by wiring at the edges of the array of ICCDs in a TDICCD sensor and are consequently controlled in common for all stages, butthe ICCD clock lines 1A, 2A, 3A, 4A, 4B, 1B, 2B, and 3B herein arecapable of being controlled independently of ICCD clock lines 1, 2, 3,and 4.

In FIG. 10, phases Φ1 and Φ2 select two different points in the ICCDs atwhich integrating pixels are permitted to be selectively reset. PhasesΦ3 and Φ4 are similarly modified to select yet another two differentpoints in the ICCDs for selectively resetting integrating pixels. FIG.11 expands FIG. 10 to show 11 ICCD stages and includes modifications tophases Φ3 and Φ4 that permit integrating pixels to be selectively resetat another two points in the ICCDs. In FIG. 11, phase Φ2 permitsintegrating pixels to be selectively reset one ICCD stage (four phases)before readout (at label A); phase Φ1 permits integrating pixels to beselectively reset two and one-quarter ICCD stages (nine phases) beforereadout (at label B); phase Φ4 permits integrating pixels to beselectively reset four and one-half ICCD stages (eighteen phases) beforereadout (at label C); and phase Φ3 permits integrating pixels to beselectively reset eight and three-quarters ICCD stages (thirty-fivephases) before readout (at label D). Assuming the ICCDs in FIG. 11include 16 stages in total, each integrating pixel delivered to the RCCDintegrates light over time selected from four phases, nine phases,eighteen phases, thirty-five phases, or sixty-four phases.

In the embodiment described above, the storage regions produced by theICCD clock lines, barrier implants, and gates have a limited capacityfor holding electrical charge. If the charge capacity of a storageregion is exceeded by an integrating pixel's charge packet, the excesscharge may spill into adjacent ICCDs or adjacent storage regions,thereby corrupting nearby integrating pixels. This spillover process istypically called “blooming.” This is a particular problem for thoseintegrating pixels associated with the brightest regions of the scene.For example, if a particular integrating pixel is selected for reset oneICCD stage before being read out of the ICCD, this means that itrequires only one stage of integration to reach a reasonable signallevel for measurement.

If there are sixteen stages in the complete ICCD, this particularintegrating pixel has probably exceeded the charge capacity of thestorage regions after integrating for only a few stages. As it continuesto collect charge after that point, the integrating pixel may causeblooming.

In order to mitigate the potential for blooming, integrating pixelsselected for reset at a particular ICCD stage are also reset at everyearlier opportunity. Using the example from the discussion of FIG. 11,if an integrating pixel is selected for reset during phase Φ2 fourphases before readout, it is also reset during phase Φ1 nine phasesbefore readout, phase Φ4 eighteen phases before readout, and phase Φ4thirty-five phases before readout.

When an ICCD gate is operated to reset an integrating pixel in aselected row of ICCD stages, the integrating pixels in the unselectedrows benefit from a measure of blooming protection provided by theoperation of the ICCD gate, as shown in FIG. 12. FIG. 12 has the samepotential profile as FIG. 7 showing the potential profile for anunselected row and an unselected ICCD gate (left ICCD) and a selectedICCD gate (right ICCD). However, while the charge packet 1202 in theleft ICCD is confined like charge packet 702 in FIG. 7, charge packet1204 in the right ICCD exceeds the charge capacity of the storageregion. Instead of blooming into neighboring storage regions, the excesscharge flows into the drain.

The ICCD gates are used selectively for blooming control in anembodiment in accordance with the invention. In this case, during aphase when none of the rows of ICCD stages are selected for possibleintegrating pixel reset, the ICCD gates are driven to a voltage topermit excess charge to be drained from the integrating pixels. FIG. 13is a modification of FIG. 7 that shows an unselected row with both ICCDgates operated to lower the barriers between the ICCDs and theirassociated drains. Charge packet 1302 in the left ICCD does not exceedthe storage region's charge capacity. Charge packet 1304 in the rightICCD exceeds the storage region's charge capacity, but the excess chargeflows harmlessly into the associated drain. In preferred embodiments ofthe invention, when the ICCD gates are used specifically for bloomingcontrol, they need not be driven to the same voltage as when they areused for selective reset of integrating pixels. This use of the ICCDgates for blooming control is analogous to a lateral overflow drain(LOD) for excess charge.

When the ICCD gates are used for blooming control as described in theprevious paragraph, all stages of the ICCDs are in the unselected stateto prevent integrating pixels from being inadvertently reset. This isachieved by splitting each phase Φ1 through Φ4 into two parts, one partin which a row of ICCD stages is selected for possible integrating pixelreset and another part in which the row of ICCD stages is deselected andduring which the ICCD gates are driven to a voltage appropriate forblooming control. Alternatively, one or more of the phases Φ1 through Φ4is used to provide an opportunity for blooming control. In FIG. 14,phases Φ1 and Φ3 each select a row of ICCD stages for possible reset ofintegrating pixels, while no ICCD stages are selected for reset inphases Φ2 and Φ4. During phases Φ2 and Φ4 the ICCD gates are driven to avoltage appropriate for blooming control as shown in FIG. 13. Althoughthis reduces the number of opportunities for integrating pixel reset, itprovides an opportunity every other phase to turn on an LOD for all theintegrating pixels in order to prevent blooming.

In another embodiment in accordance with the invention, two modes ofoperation are enabled. In one mode of operation, the ICCD clock linesand gates are operated to provide selective reset of integrating pixelsand, optionally, blooming control. In a second mode of operation, theICCD clock lines are operated only to permit charge transfer along theICCD; no rows of ICCD stages are selected for possible reset ofintegrating pixels. Also in the second mode of operation, the ICCD gatesoptionally are operated in order to enable an LOD for bloomingprotection. This second mode of operation is useful when the scene to becaptured has a low light level and a limited dynamic range, so that allintegrating pixels are integrated over the full length of the ICCDs andnone are reset. In this case, the charge capacity of the storage regionsin the ICCDs is increased by maintaining the voltage of the ICCD gatessuch that the potential barrier produced by the gate is greater than thebarrier produced when the gate is used for selecting an integratingpixel for reset.

In another embodiment in accordance with the invention, the ICCD gatesare separated into individual gates that are associated with individualICCD stages or with individual ICCD clock lines within the individualstages. Control of the gates is provided by circuitry on anotherintegrated circuit that is connected to the gates by wafer-to-wafer ordie-to-die bonding. For example, individual ICCD gates are associatedwith the ICCD clock lines 2 and 3 in FIG. 5 in each stage of each ICCD.During phase Φ2 in FIG. 5, charge packets associated with integratingpixels selected for reset are allowed to flow into the drain associatedwith the ICCD by operating the corresponding individual gates. Sincecontrol of the gates is provided by separate circuitry, the gates may becontrolled independently and there is no need for selecting a row ofICCD stages for possible reset of integrating pixels.

FIG. 15 shows the operation of individual gates for each ICCD asdescribed in the previous paragraph. FIG. 15 is similar to FIG. 8 inthat it shows the potential profile across two ICCDs along clock line 2,but clock line 2 is in its conventional storage state, not in a state toselect a row of ICCD stages. The individual gate associated with theright ICCD is operated to allow the charge packet 1504 associated withintegrating pixel in the right ICCD to flow into the drain. Theindividual gate associated with the left ICCD is set to a point thatprovides blooming protection, so that excess charge in the charge packet1502 is allowed to flow into the drain instead of blooming.

Providing individual gates for the ICCD stages allows increasedflexibility in operation. For example, for integrating pixels that areselected to be reset in order to reduce their integration time, thoseintegrating pixels are reset repeatedly as they pass through each stageof the ICCD.

FIG. 16 is a plan view of portions of ICCDs of a TDI CCD in anotherembodiment of the present invention. ICCD clock line 1 (502), ICCD clockline 2 (504), ICCD clock line 3 (506) and ICCD clock line 4 (508) areoriented horizontally in FIG. 16, which also depicts channel stops 1602and 1604. A sense node 1606 (e.g., a floating diffusion nodeelectrically isolated from other nodes in the device and where receivedphotocharge may be converted to a voltage) is located between channelstop 1602 and channel stop 1604.

A contact 1608 to the sense node 1606 permits the charge in the sensenode 1606 to be measured and permits the sense node to be reset bycircuitry as will be described below. A gate 1610 is operable indifferent modes to provide a charge-spillover threshold for the adjacentICCD integrating channel and to clear charge from the adjacent channel.As in FIG. 4 discussed above, FIG. 5 represents a cross-section of avertical portion of one of the ICCDs in FIG. 16 as shown by the cut line5-5 in FIG. 16. FIG. 17 is a cross-section that cuts horizontallythrough the ICCD of FIG. 16 as shown by cut line 17-17 in FIG. 16.

The cross-section of FIG. 17 cuts along ICCD clock line 2 (504) acrossan ICCD constructed in accordance with an embodiment of the presentinvention. Channel stop implants 1604 in FIG. 17 produce potentialbarriers to separate each ICCD from adjacent ICCDs. Sense node implant1606 provides a region that is resettable to permit the collection andmeasurement of spillover charge from the ICCD integrating channel aswill be described below in more detail. Gate 1610 produces a potentialregion that provides an adjustable barrier between an ICCD integratingchannel and its corresponding sense node implant 1606.

FIGS. 18-21 are potential diagrams associated with FIG. 17 for an ICCDin accordance with an embodiment of the invention. In FIGS. 18-21, theICCD clock lines are driven to produce the potentials associated withpotential diagram Φ2 in FIG. 5 so that a storage region to hold a chargepacket associated with an integrating pixel is in the ICCD integratingchannel adjacent to gate 1610.

In FIG. 18, the gate 1610 is held in a state that holds integratingpixel charge packet 1802 in the integrating channel of the ICCD. Thesense node 1606 adjacent to gate 1610 has been reset by momentarilyconnecting it to ground in order set the sense node potential to a knownstate. FIG. 18 also depicts a charge packet 1804 in a neighboringstorage region.

The charge packet 1802 is held by the storage region potential profilesproduced by the left channel stop 1604, ICCD clock line 1 (502), ICCDclock line 4 (508), channel stop 1602, and gate 1610. The storage regionproduced by these elements has a limited capacity for holding electricalcharge. In FIG. 18, the charge packet 1802 is less than the chargecapacity of the storage region. Conversely, the charge packet 1902 inFIG. 19 exceeds the charge capacity of the storage region. The excesscharge from charge packet 1902 is allowed by the potential produced bythe gate 1610 to spill into the sense node 1606, as shown in FIG. 19 (asshown, neighboring charge packet 1904 is unaffected by such a spillover). Charge that spills into the sense node 1606 is termed “spillovercharge.” The potential produced by the charge in sense node 1606 may bemeasured by a measurement circuit, or the sense node 1606 may be resetin order to remove the spillover charge, or both actions may be done insequence in embodiments of the present invention.

In FIG. 20 the gate 1610 is operated to eliminate the barrier betweenthe storage region and the sense node 1606, allowing the charge 2002 inthe storage region to flow into the sense node 1606 (again with noimpact on neighboring charge packet 2004). This permits integration tobe restarted for the integrating pixel in the integrating channel of theICCD. If the sense node has sufficient charge capacity it is capable ofholding all of the charge from the storage region. Alternatively, thecharge is removed by resetting the sense node.

FIG. 21 shows the state of the sense node 1606 and the storage regionafter removing the charge from the storage region and sense node 1606 byoperating the gate 1610 as in FIG. 20, resetting the sense node, andrestoring the gate 1610 to its barrier state (as in FIGS. 18-20,neighboring charge packet 2104 remains in its storage region). At thispoint, photocharge begins to collect in the storage region (resultingin, e.g., the state illustrated in FIG. 18). Resetting the integratingpixel in this way effectively reduces the available integration time forthe reset integrating pixel to be the time spent in the remaining ICCDstages between where the integrating pixel reset occurred and the end ofthe ICCD.

By providing each stage of the ICCD with a gate and a sense node asdescribed above, any integrating pixel being clocked along an ICCD maybe reset at any ICCD stage by the operation of the gate and sense nodeas described with respect to FIGS. 18-21. The selective reset ofintegrating pixels provided by this arrangement enables integratingpixels associated with bright regions of the scene to be reset at apoint close to the end of the ICCD, thereby permitting them to have ashort effective integration time. In a similar fashion, integratingpixels associated with middle ranges of scene brightness are enabled tobe reset in the middle of the ICCD, permitting them to have a mediumlength effective integration time. Integrating pixels associated withdark areas of the scene do not have to be reset at all, permitting themto have the maximum integration time allowed by all stages of the ICCD.This has the effect of significantly increasing the dynamic range of theTDI image sensor.

The embodiment described above provides the advantage of increaseddynamic range without the shortcomings of earlier approaches. Forexample, compared to conventional charge measurement circuitry, the gateand sense node associated with an ICCD stage are compact structures. Allof the ICCD stages are identical in the disclosed embodiment, unlike theconventional stages that incorporate specialized charge measurement anddischarge circuitry. Embodiments of the present invention also do notrequire the placement of a contact in the ICCD for charge measurement,thereby eliminating the possibility of disrupting the integrating pixelwith dark current or other undesirable effects. Finally, since the gateand sense node are externally controllable, the entity that controlsthese elements and thereby causes the reset of integrating pixels hasknowledge of which integrating pixels were reset and at which ICCD stagethey were reset. Consequently, in contrast to the output pixels producedby conventional sensors, there is no ambiguity regarding the outputpixels: since the reset conditions for a given integrating pixel areknown, the integration time for the corresponding output pixel is alsoknown.

As was described previously with respect to FIGS. 18 and 19, the storageregions produced by the ICCD clock lines, barrier implants, and gateshave a limited capacity for holding electrical charge. If the chargecapacity of a storage region is exceeded by an integrating pixel'scharge packet, the excess charge may spill into adjacent ICCDs oradjacent storage regions, thereby corrupting nearby integrating pixels(which, as detailed above, is typically referred to as “blooming”). Thisis a particular problem for those integrating pixels associated with thebrightest regions of the scene. For example, if a particular integratingpixel is selected for reset one ICCD stage before being read out of theICCD, this means that it requires only one stage of integration to reacha reasonable signal level for measurement. If there are sixteen stagesin the complete ICCD, this particular integrating pixel has probablyexceeded the charge capacity of the storage regions after integratingfor only a few stages. As it continues to collect charge after thatpoint, the integrating pixel may cause blooming.

In order to mitigate the potential for blooming, integrating pixelsselected for reset at a particular ICCD stage may be reset at everyearlier opportunity, as also described above. For example, if anintegrating pixel is reset one ICCD stage before readout, it is alsoreset in every earlier ICCD stage it passes through that includes a gateand sense node. Alternatively, the gate is used selectively for bloomingcontrol in an embodiment in accordance with the invention. In this case,the gate is set to a level that sets a threshold for charge that islower than the other storage region barriers, causing excess charge tospill preferentially over the gate barrier into the sense node. This useof the gate for blooming control produces a lateral overflow drain forexcess charge. Note that the threshold for spillover charge isadjustable depending on the voltage applied to the gate.

The discussion to this point relates to using the gate and sense nodefor resetting an integrating pixel or for providing blooming protection.The gate and sense node also provide a way to measure the spillovercharge. This measurement is used to make decisions regarding whichintegrating pixels to reset and at which ICCD stage to set them.Alternatively, the spillover charge measurements are used to augment themeasured integrating pixels.

FIG. 22 shows a spillover charge measurement circuit 2200 in accordancewith an embodiment of the invention. Sense node 1606 is electricallyconnected to reset and readout circuitry by a conductor 2202. Thevoltage of gate 1610 is controlled by a conductor 2204 and a terminal2206. The sense node 1606 is connected to a source follower 2208. Areset gate 2210 is operated by a terminal 2212 in order to connect thesense node 1606 to ground to establish a known reset state for the sensenode 1606. Sense node 1606 converts spillover charge into a voltage, aprocess that depends on the capacitance of the sense node 1606: a smallcapacitance produces a higher voltage than a large capacitance for thesame amount of charge. The voltage at the sense node 1606 is applied tothe gate of source follower 2208 in order to buffer the source followervoltage and provide drive for an output terminal 2214. Optional selectgate 2216 is operated by a terminal 2218 in order to either connectsource follower 2208 to commonly used readout circuitry or to isolate itfrom other source followers that share the same readout circuitry. Thecircuitry 2200 may be fabricated in the same substrate as the TDI CCDand associated gate and sense node with which it is associated, or itmay be fabricated in a different substrate that is electricallyconnected to the gate and sense node in the TDI CCD substrate byconductors 2204 and 2202 (as indicated by interface 2220). Suchconnection may be achieved by wafer-to-wafer or die-to-die bonding inways well-known to those skilled in the art.

Measurement of spillover charge begins with gate 2216 being turned on byterminal 2218 to connect source follower 2208 to output terminal 2214.Sense node 1606 is reset to a known state by momentarily turning onreset gate 2210 with terminal 2212. After sense node 1606 is reset andreset gate 2210 is turned off, the voltage at output gate 2214 issampled and held; this is the reset sample. After a period of time tocollect spillover charge in the sense node 1606, the voltage at outputgate 2214 is sampled and held again; this is the signal sample. Themeasurement of the spillover charge is the difference between the signalsample and the reset sample. The reset, reset sample, spilloveraccumulation time, and signal sample may all occur during phase Φ2(FIGS. 5, 16, and 17), or reset and reset sample may occur during phaseΦ4 when the gate is adjacent to a barrier region and signal sample mayoccur during subsequent phase Φ2 when the gate is adjacent to a storageregion. Although the described embodiment of spillover chargemeasurement employs correlated double-sampling in a manner well-known tothose skilled in the art, a measurement may also be done with only thesignal sample, although the result may have somewhat higher error. Thecapacitance of the sense node 1606 may be made very small so smallamounts of spillover charge cause large voltage swings at the sense node1606; this effectively provides a high gain measurement of the spillovercharge.

The measured spillover charge is used for any of several purposes inembodiments in accordance with the invention. For example, the chargespillover measurements associated with the passage of an integratingpixel along the ICCD may be stored and used along with the measuredintegrating pixel to determine a final extended dynamic range pixel.Integrating pixels associated with dark areas of a scene to be capturedwill typically have no spillover charge, so the spillover chargemeasurements will be zero and the measured integrating pixel aloneprovides the result. Integrating pixels associated with bright areas ofthe scene may have significant amounts of spillover charge beginningsoon after the integrating pixels begin travelling along the ICCD. Thecombination of the spillover charge measurements for a brightlyilluminated integrating pixel and the measurement of the integratingpixel itself provides the final pixel value. Integrating pixelsassociated with mid-range brightness areas of the scene will typicallyhave modest amounts of charge spillover that begins after some number ofICCD stages have been passed by the integrating pixels. In this case,the non-zero spillover charge measurements for a mid-range illuminatedintegrating pixel and the measurement of the integrating pixel itselfprovides the final pixel value. In order to avoid accumulating noisefrom spillover charge measurements in the determination of the finalpixel, a threshold may be applied to the spillover charge measurementsto determine whether or not a particular measurement should be used.

The spillover charge measurement may also be utilized to determine whichintegrating pixels to reset and at which stage of the ICCD to resetthem. For example, if spillover charge is detected after an integratingpixel has traversed the first two stages of an ICCD, then the lightlevel for that integrating pixel is such that the integrating pixelrequires only a single stage of integration. Consequently, theintegrating pixel is reset through every subsequent stage of the ICCDuntil it reaches the final stage, at which point it is allowed tointegrate through the final stage. Similarly, if spillover charge isdetected after an integrating pixel has traversed the first six stagesof an ICCD, then the light level for that integrating pixel is such thatthe integrating pixel will produce a strong, but not overflowing, signallevel after five stages of integration. In this case, the integratingpixel is reset through every subsequent stage of the ICCD until itreaches the fifth to last stage, at which point it is allowed tointegrate through the final five stages. Note that the number of stagesof integration determined and used for each integrating pixel isreported by the entity that makes the determination (e.g., thecontroller) so that it may be used along with the measured integratingpixel to product the final pixel.

When charge spillover detection is used to determine the number ofstages used for integrating each integrating pixel as described in theprevious paragraph, an issue may arise if spillover is detected onlyafter an integrating pixel passes the halfway point in the ICCD. In thiscase, the required number of stages for integration is less than thefull number in the ICCD but more than the number of stages alreadypassed by the integrating pixel. This is addressed by adjusting thepotential produced by the gate to allow spillover at a lower level ofcharge in earlier stages of the ICCD. For example, by setting the gatethreshold after the first stage of a 16-stage ICCD to 1/16 the maximumcharge capacity of a storage region of the ICCD, any integrating pixelsthat do not cause charge spillover of that lower gate threshold canintegrate for the full 16 stages without causing charge spillover. Ifcharge spillover occurs, the measured charge spillover is used todetermine how many stages of integration are required, provided thecharge spillover does not saturate the sense node 1606. If the chargespillover saturates the sense node, then the next stage of the ICCD hasits gate adjusted to allow spillover at a higher amount of charge,thereby refining the measurement of spillover charge for thoseintegrating pixels that saturated the measurement at the first stage.Such adjustment of gate thresholds and detecting and measuring spillovercharge may be used in a successive-approximation fashion to determinethe number of final stages through which each integrating pixel shouldbe run, resetting the integrating pixel up until that point. Variationsin gate thresholds due to manufacturing or other variations may requirecalibration or application of calculation thresholds when using thespillover charge measurements in determining a final pixel value or whenused for spillover charge detection. Although the transistors andreference levels described above have been NMOS transistors andreference levels suitable for operation with an electron-collecting CCDarrangement, PMOS transistors and corresponding reference levels may beused with a hole-collecting CCD arrangement.

Although mechanisms for determining which integrating pixels to resetand at which ICCD stage to reset them have been described above, thedetermination may also be based in whole or in part on a prediction ofscene content. The prediction may be made either because the scenecontent is well controlled and may be accurately anticipated or becausean earlier image of the scene has been captured and upon which theprediction is based. Alternatively, a separate sensor may be arrangedpreceding the TDI sensor in the scanning process, thereby providing acaptured image of the scene that is closely followed by capture of thesame scene by the TDI sensor. The earlier captured image of the scenemay have a reduced dynamic range or increased noise compared to the TDIcapture, but the earlier captured image is sufficient to determine whichintegrating pixels to reset and at which ICCD stage to reset them in theTDI capture. There may be a one-to-one correspondence between pixels inthe earlier captured image and the integrating pixels in the TDIcapture, allowing the determination regarding integrating pixel reset tobe made based on the corresponding pixel in the earlier captured image.Alternatively, the earlier captured image may have multiple pixelscorresponding to the integrating pixels in the TDI capture, allowing thedetermination regarding integrating pixel reset to be made on the basisof several corresponding pixels in the earlier captured image. In yetanother alternative, each pixel of the earlier captured image maycorrespond to several integrating pixels in the TDI capture, with thedetermination of integrating pixel reset being made collectively forsmall clusters of integrating pixels or being made on the basis of aninterpolation of the pixels of the earlier captured image.

FIG. 23 illustrates yet another embodiment in accordance with theinvention in which the dynamic range of the TDI CCD sensor is enhancedvia the selective blocking of portions of the scene to be imaged(preferably bright areas of the scene). In FIG. 23, light from a sceneelement 2302 is collected by an optical system 2304 in order to producean optical image on the face of a transmissive optical mask array 2306.The optical mask array 2306 may be either in contact with or in veryclose proximity to a TDI image sensor 2308. The optical mask array 2306may be an array of switchable optical elements, each of which may beswitched between an optically transmitting state and a partially orcompletely optically blocking state. Since TDI image sensor 2308 isused, the optical image moves with respect to the TDI image sensor 2308and the optical mask array 2306, and the TDI image sensor 2308 isclocked concurrently with the movement of the optical image.Simultaneously with the clocking of the TDI image sensor 2308 and themovement of the optical image, the switchable optical elements of theoptical mask array 2306 are switched in order to match the motion of theoptical image and the clocking of the TDI image sensor 2308. In thismanner, the optically attenuating or blocking mask 2306 may be utilizedto track a bright area of the scene, thereby preventing at least aportion of the bright light from reaching the TDI image sensor 2308. Forexample, the light from a bright area of the scene may be controlled byusing the switchable optical elements of optical mask array 2306 toprovide a synchronously travelling optical mask that blocks light fromreaching the TDI image sensor 2308 for part of the time that thecorresponding integrating pixels in the TDI image sensor 2308 are beingclocked through the ICCDs of the TDI image sensor 2308. Knowledge ofwhich pixels in the image captured by the TDI image sensor were affectedby the optical mask is combined with the pixels of the captured image inorder to provide an extended dynamic range image. (That is, the level oflight attenuation due to the mask array 2306 may be utilized todetermine the actual brightness level of the masked pixel after imagecapture.) In various embodiments of the present invention, the opticalmask array 2306 is partially attenuating for the full time thatcorresponding integrating pixels travel along the ICCDs of the TDI imagesensor, fully attenuating for part of the travel time, or a combinationof partially attenuating and fully attenuating for the travel time.

The optical mask array 2306 in FIG. 23 may be fabricated in any ofseveral different ways. In one embodiment of the present invention, theoptical mask array 2306 is made by employing liquid crystal technology.The liquid crystal material, in conjunction with optical polarizers andX-Y addressing and control schemes commonly used forliquid-crystal-based displays, provides a traveling attenuator that isadjustable over a wide range of attenuation. In another embodiment ofthe present invention, optical mask array 2306 is made usingmicro-electromechanical systems (MEMS) to provide individual mechanicalshutters that open and close to selectively allow or prevent light fromreaching the TDI sensor 2308.

The individual switchable optical elements in the optical mask array2306 may have a one-to-one correspondence with CCD elements in the TDIimage sensor 2308, there may be multiple switchable optical elements inthe optical mask array 2306 for each CCD element, or there may bemultiple CCD elements for each switchable optical element. Additionally,the optical mask array 2306 may be in intimate contact with the TDIimage sensor 2308 to cast sharply defined images of the switchableoptical elements onto the surface of the TDI image sensor, or it may beplaced in a slightly defocused position in order to soften the edgesbetween the switchable optical elements.

Another embodiment of the present invention that employs an alternativeoptical arrangement is shown in FIG. 24. In this arrangement, light fromthe scene element 2302 is collected by optical system 2304 in order toproduce an optical image on the face of transmissive optical mask array2306. The image transmitted through the optical mask is then collectedby a relay lens 2400 in order to produce a masked optical image on theface of the TDI image sensor 2308. In other regards this arrangementtypically operates the same way as the arrangement of FIG. 23.

Another embodiment of the present invention that employs an alternativeoptical arrangement is shown in FIG. 25. In this arrangement, light fromthe scene element 2302 is collected by optical system 2304 in order toproduce an optical image on the face of a reflective optical mask array2500. The image reflected by the optical mask is then collected by relaylens 2502 in order to produce a masked optical image on the face of theTDI image sensor 2308. In one embodiment of the present invention, thereflective optical mask array 2500 is fabricated by placing a reflectivesurface (e.g., a mirror) behind the transmissive optical mask array 2306of FIGS. 23 and 24. In another embodiment, the reflective optical maskarray 2500 employs a micro-mirror array (another type of MEMS structure)in which the array elements are mirrors that are individually switchableeither to reflect light toward the TDI image sensor 2308 or to reflectlight away from the TDI image sensor 2308.

As mentioned previously, the determination of the integrating pixels tobe optically masked may be based a prediction of scene content. Theprediction may be made either because the scene content is wellcontrolled and may be accurately anticipated or because an earlier imageof the scene has been captured and upon which the prediction is based.Alternatively, a separate sensor may be arranged preceding the TDIsensor in the scanning process, thereby providing a captured image ofthe scene that is closely followed by capture of the same scene by theTDI sensor. The earlier captured image of the scene may have a reduceddynamic range or increased noise compared to the TDI capture, but theearlier captured image is sufficient to determine which integratingpixels require optical masking. There may be a one-to-one correspondencebetween pixels in the earlier captured image and the integrating pixelsin the TDI capture, allowing the determination regarding optical maskingto be made based on the corresponding pixel in the earlier capturedimage. Alternatively, the earlier captured image may have multiplepixels corresponding to the integrating pixels in the TDI capture,allowing the determination regarding optical masking to be made on thebasis of several corresponding pixels in the earlier captured image. Inyet another alternative, each pixel of the earlier captured image maycorrespond to several integrating pixels in the TDI capture, with thedetermination of optical masking being made collectively for smallclusters of integrating pixels or being made on the basis of aninterpolation of the pixels of the earlier captured image.

The sensor that provides the earlier captured image generally precedesthe TDI image sensor in the scanning process, and may therefore betermed a “leading sensor.” FIG. 26 depicts a TDI CCD image sensor 2600with a leading linear array image sensor 2602. Light from a sceneelement 2604 is collected by an optical system 2606 in order to producean optical image 2608 that extends across the face of the TDI imagesensor 2600 and also on the leading linear array image sensor 2602.Scene element 2604 moves vertically upward with respect to opticalsystem 2606 and the two image sensors 2600, 2602, causing correspondingoptical image 2608 to move vertically downward across the linear arrayimage sensor 2602 and the TDI image sensor 2600. Since each element ofthe image is first encountered by the leading image sensor 2602, thecaptured image from the leading image sensor 2602 may be utilized tomake decisions regarding the operation of the TDI image sensor 2600and/or an optical mask utilized therewith, as discussed above.

The leading image sensor, in addition to its use as a predictive sensor,is used in one embodiment of the invention to provide additional sceneinformation. For example, the additional scene information includescolor information if the TDI sensor does not capture color information.(For example, portions of the leading sensor may incorporate colorfilters each passing light of a particular color or portion of theelectromagnetic spectrum, and the photocharge generated therein may thusprovide color information for the captured scene, and the TDI sensor maynot incorporate such color filters and may thus collect only monochromeintensity levels related to the scene.) By way of example, the leadingimage sensor may be a linear array sensor, a multiple linear arraysensor, an area array sensor, or a TDI sensor. Also by way of example,the leading image sensor may be separate from the associated TDI sensoror may be integrated into the same integrated circuit substrate as theTDI sensor.

A trailing image sensor, one that is preceded in the scanning process bythe TDI sensor (but otherwise resembles the leading image sensordescribed above), may be used to provide a later captured image. Thelater image may be used in conjunction with an earlier captured image todetermine that no significant changes occurred in the scene between thetime the earlier captured image was captured and the time that latercaptured image was captured. Changes detected in the scene between theearlier and later captured images may be used to flag pixels in the TDIcaptured image as potentially being under- or over-exposed because of anincorrect TDI captured image prediction and/or because of an incorrectoptical masking determination based on the earlier captured image.

The foregoing description of a TDI CCD with selective pixel integrationperiod will be understood by one skilled in the art. Variations andmodifications may be effected within the spirit and scope of theinvention. By way of example, a different number of phases may be usedto clock charge packets along the ICCD, barrier implants may be used inthe ICCDs to permit two-phase operation, and implants may be used toadjust gate threshold levels. In another example, the RCCD is replacedwith alternative readout circuitry, either on the same substrate as theICCDs or on a separate substrate. All of these variations and otherscontemplated or made by one skilled in the art remain within the scopeof the invention.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. An imaging system comprising: atime-delay-and-integrate (TDI) image sensor comprising (i) a pluralityof integrating CCDs (ICCDs), arranged in parallel, that accumulatephotocharge in response to exposure to light, (ii) electrically coupledto the plurality of ICCDs, a readout CCD (RCCD) for receivingphotocharge from the plurality of ICCDs, and (iii) electrically coupledto the RCCD, readout circuitry for converting charge received from theRCCD into voltage; an optical system for receiving light from a scene tobe imaged and projecting it on the plurality of ICCDs; and disposedbetween the optical system and the plurality of ICCDs, an optical maskfor selectively altering an intensity of light projected to at leastportions of the ICCDs.
 2. The imaging system of claim 1, wherein theoptical mask comprises an array of independently controllable maskingelements each for attenuating light collection by a different portion ofthe ICCDs.
 3. The imaging system of claim 1, wherein the optical maskcomprises an array of independently controllable reflective elementseach for selectively reflecting a portion of the light from the opticalsystem onto the ICCDs.
 4. The imaging system of claim 1, furthercomprising a control system for controlling the optical mask based atleast in part on light from the scene to be imaged before such light isprojected by the optical system.
 5. A method of image capture utilizinga time-delay-and-integrate (TDI) image sensor comprising (i) a pluralityof integrating CCDs (ICCDs), arranged in parallel, that accumulatephotocharge in response to exposure to light, (ii) electrically coupledto the plurality of ICCDs, a readout CCD (RCCD) for receivingphotocharge from the plurality of ICCDs, and (iii) electrically coupledto the RCCD, readout circuitry for converting charge received from theRCCD into voltage, the method comprising: projecting light received froma scene to be imaged onto the plurality of ICCDs to capture an image ofthe scene; and during capture of the image, selectively altering anintensity of light projected to at least portions of the ICCDs.
 6. Themethod of claim 5, wherein the intensity of light projected to at leastportions of the ICCDs is altered with an optical mask disposed betweenthe scene and the ICCDs.
 7. The method of claim 6, wherein the opticalmask comprises an array of masking elements each independentlycontrollable to mask a portion of the ICCDs whereby light collection inthe masked portion is attenuated.
 8. The method of claim 6, wherein theoptical mask comprises an array of reflective elements eachindependently controllable to reflect a portion of the projected lightonto the ICCDs.
 9. The method of claim 5, wherein the selectivealteration of the intensity of light projected to at least portions ofthe ICCDs during capture of the image is based at least in part on apreviously captured image.